Objective: To further elucidate results observed in mice of different inbred strains painted over flanks and back with 3- methylcholanthrene (MCA) or untreated. An association has been noted between resistance to the induction of skin tumors by MCA and high susceptibility to MCA-induced leukemia, or to spontaneous leukemia induced by endogenous murine leukemia virus (MuLV). Thus, depending on the strain, mice develop a high incidence of only 1 of these 3 neoplastic conditions: BAlB/c mice of MCA-induced skin tumors; RF/J mice of MCA- induced leukemia; AKR mice of spontaneous leukemia. Approach: Experiments using genetic techniques are in progress in the attempt to identify the factors responsible for each mouse strain developing only 1 of the 3 neoplastic conditions observed. The experiment involves MCA-painted or untreated F1 hybrid mice of the cross BALB/c X RF/J mice or BALB/c X AKR mice.